The invention relates to an endoprosthesis.
With extensive osseous defects, for example an ostectomy caused by a tumor, it is possible to retain most of the function of the respective limbs and to avoid an otherwise necessary amputation by using an endoprosthesis that balances out the existing bone loss.
The German Unexamined Published Patent No. DE 35 28 728 discloses an endoprosthesis for replacing the center section of an elongated bone with a sleeve-shaped spacing element, having a stationary pin on one end and a pin that is arranged axially displaceable inside the spacing element on the other end.
The above-described endoprosthesis has the disadvantage of requiring a relatively large distance between the bone fragments to be joined, despite its two-piece construction. Such a large extension frequently cannot be realized at all or only by causing a correspondingly extensive trauma to soft tissues, muscles or even tendons. In addition, the prosthesis provides only for a single screw-together option to secure the axially movable pin in a selected position against an undesirable axial displacement. The frictional connection necessary between the wall of the spacing element and the pin, arranged therein so as to be displaceable, to secure the desired pin position can be produced only with difficulties under the conditions of an operation.
Starting with the shortcomings in the state of the technology, it is the object of the invention in particular to specify an improved endoprosthesis. For one thing, this endoprosthesis is distinguished by a great variability in length, leads to further simplifications in operating technology and can be inserted with less trauma to the body part to be treated operatively.
The invention includes the realization that the better the adaptation of the endoprosthesis to the location of use, the lower the trauma to the body tissue during the surgical implantation of a prosthesis. Of particular advantage in this case are prostheses composed of several individual parts for which the size can be varied.
The invention furthermore includes the provision that for an endoprosthesis with pin-type securing means and a two-part, sleeve-shaped intermediate member as replacement for parts of elongated bones, the end of the rotation-symmetrical pin through a frictional connectionxe2x80x94if necessary also a frictional and form-fitting connectionxe2x80x94in the two-part sleeve, an axial displacement of the pin is precluded, even for higher stresses in axial direction. The frictional connection is generated by joining the two sleeve parts, preferable, by screwing them together. Prior to combining the sleeve parts, the total length of the endoprosthesis can be varied without problems through an axial displacement of the securing means protruding on one side from the intermediate member.
Specially adapted end regions of the intermediate member in the form of half shells, having a reduced diameter as compared to the remaining inside diameter, function in the manner of clamping jaws. These encircle and immobilize pin-shaped securing means in the manner of a clamp, so that the securing means can be locked into different positions. Depending on the conditions, the securing means can be secured in a more or less extended position. A rotational adjustment is also possible in the process, which is particularly important if the securing means are curved and an additional directional adjustment can be made with the rotation. Furthermore, a corresponding inside profiling of the half shells advantageously permits an adaptation to different cross sections of the securing means. The adaptation is aided by designing the half shell inside to have limited resiliency, so that an additional form adaptation can occur during the clamping operation, which results in an improved holding effect.
Special advantages can be achieved, in particular, with a local material deformation, caused by characteristic differences in the materials for the sleeve parts and the pins, which results in an increase in the surface areas that are operatively connected at the anchoring location.
In accordance with the preferred embodiment of the invention, the endoprosthesis for replacing osseous areas in the diaphysial and metaphysial region of an elongated bone comprises a two-part intermediate member that is divided in the direction of its longitudinal axis. The intermediate member has an essentially cylindrical design and can be assembled by fitting together two identically shaped half shells. Each half shell contains a semi-cylindrical recess at the end for forming a passage at both ends after the intermediate member is assembled. Pin-shaped or nail-shaped means for the intramedullary securing of the intermediate member are provided in these passages, in such a way that prior to joining the parts of the intermediate member, they can be advantageously inserted and positioned in the passage, so as to be axially displaceable. Subsequently, the securing means are anchored in the selected position inside the passages by joining the half shells, wherein the necessary frictional connection is generated preferably by screwing together the half shells. As a result, a dimensionally correct adaptation of the endoprosthesis to the patient-specific conditions is easily possible without causing excessive trauma to the area of the body that is to be treated.
In accordance with the preferred embodiment of the invention, the wall of the semi-cylindrical recess is provided with a profiling at the ends of the part elements that can be fitted together to form the intermediate member to obtain a secure anchoring of the securing means. It is advantageous to use beads for this profiling, which extend in a semi-circular shape and are arranged in rows as seen in axial direction. Another embodiment of the invention provides that inserting a semi-cylindrical sleeve part into each of the recesses of the part elements is advantageous for a possible subsequent operation. These inserted, semi-cylindrical sleeve parts are provided with a profiling of semi-circular beads, arranged in rows in axial direction and can be easily replaced in case of wear. To secure the sleeve part against an axial displacement, for example, the recess contains an annular groove. A ring-shaped bead, arranged at the periphery of the sleeve part, engages in this groove.
One advantageous modification of the invention provides that the annular beads of the profiling on the inside walls of the passages, formed when the half-shells of the intermediate member of the endoprosthesis are joined, as well as of the beads for the semi-cylindrical sleeve parts have a trapeze-shaped or triangular, respectively an essentially semi-circular cross-sectional profile. A deformation of either the securing means shank which is positioned inside a passage, or a deformation of the annular beads on the passage wall is favorably ensured in this way, depending on the strength ratio of the materials used for the securing means and the shell-shaped part elements of the intermediate member of the endoprosthesis. Such a deformation results in each case in an enlargement of the surface areas for the operational engagement of the individual parts of the endoprosthesis according to the invention and thus to an increase in the desired frictional connection for anchoring the securing means.
According to another modification of the invention, it is favorable if the semi-circular beads of the recesses forming the passages or of the semi-cylindrical sleeve parts are divided into individual segments in tangential direction to ensure a good frictional and form-fitting connection between intermediate member and securing means.
Based on another embodiment of the invention, the shank for the intradmedullary securing means has a profiling, which corresponds essentially to the profiling provided in the passages of the intermediate member of the endoprosthesis. This design of the securing means permits a combination frictional and form-fitting connection when the part elements of the intermediate member with inserted securing means are joined. The endoprosthesis is adapted gradually to the size of the bone defect to be surgically corrected in each case, wherein the possibilities for an exact adaptation are determined by the respective size dimensions of the profiling.
Independent of the configuration of the shank for the securing means, which must be anchored in the intermediate member, grooves are provided along the shank periphery, which extend in axial direction over the total length of the securing means and are arranged in a radial-symmetrical distribution. The grooves increase the safety against turning of the securing means, which are inserted intramedullary into the bone segments to be joined.
In accordance with another advantageous embodiment of the endoprosthesis according to the invention, the securing means are designed as intramedullary nails that are preferably commercially available and have a hollow body shape. They contain a longitudinal slot, extending over a section of the nail wall or over the complete nail length. One advantageous modification of the invention provides that the end region, which can be inserted into one passage of the intermediate member, preferably consists of a solid material that allows a particularly secure anchoring in the medulla of the bone segments to be joined. On the one hand, the longitudinal slot ensures a radial elasticity. On the other handxe2x80x94promoted by a growing-in of the bone materialxe2x80x94it provides sufficient security against turning of the intramedullary anchored securing means. Depending on the situation, it may be advantageous to insert curved securing means for the purpose of adaptation to the position and/or form of the bone sections that must be joined to form a complete bone by means of the endoprosthesis according to the invention. The end regions of the securing means to be anchored inside the intermediate member do not have this essentially circular-arc shaped curvature.
To accelerate the growing-in of the endoprosthesis into the surgically treated area of implantation, a biocompatible coating is applied to the outer surface of the part elements that are joined to form the intermediate member. In addition, the surface on the end faces of the part elements contains a scarring formed by protuberances, wherein the protuberances, for example, have a hemispherical shape. The walls of the part elements are provided with essentially circular openings.
In accordance with another modification of the invention, the sides of the part elements that face each other in the assembled state of the intermediate member are provided with latching means in the form of pegs and holes, which make it possible to secure the position of the part elements prior to the joining.
The individual parts for the endoprosthesis fitted together according to the invention are fashioned from a body-compatible and body-resistant, preferably metallic material such as titanium, tantalum, niobium or corresponding alloys.